text:
Can I use motorcycle engine in aircraft
I am trying to build an Aircraft, The weight of the Airplane will be around 40 Kg with engine and 8 liters of fuel and with me the total weight will be around 90 to 100 kg and I want to use a motorcycle engine in the aircraft, the aircraft will be one seater and I have a question that which type of engine is best to use,

Displacement    123 cc
Engine Type 4 Stroke, Air-Cooled, SOHC
Max Power   10.7 bhp @ 7500 rpm
Max Torque  10.40 Nm @ 6500 rpm

The engine produces a maximum peak output power of 11.00 HP (8.0 kW) @ 9000 RPM and a maximum torque of 9.80 Nm (1.0 kgf-m or 7.2 ft. lbs) @ 7000 RPM
So should I use 1070 or 1065 size wood propeller and if this size is not suitable so please recommend me the best size,
If this plane successfully flies so what would be speed of it and will it give me same mileage as the engine gives in Motorcycle? And for how long I can fly nonstop with these engines,
Mine Airplane design would be like this

Please explain me as simple as possible, and Sorry for Bad English. thanks

The common Cessna 172 has a weight of over 1 ton and 160 horsepower.
160/1111=around 0.15
Your aircraft:
11/100=0.11

Don't get me wrong though, the Cessna 172 carries 3 people at around 10,000 feet;you only want yourself to fly, possibly lower, where the air is thicker and planes fly easier.

So, will my plane fly easily?

no, it will be marginal. High span will help but you will need struts or you exceed your weight goal.

won't that be an interesting exercise for him - designing a large and light wing :D

Hey @PeterKämpf how about this - use his motor to drive a compressor for a turbojet :D

Should we go to chat?

You mean like 100 years ago when Indian motorcycle engines were used in amateur planes ?

When these things are on a motorcycle, the top speed is around 50-60mph. Even two of these together doesn't leave you much margin. Motorcycle engines are not a good fit because the integrated gearbox is useless and they are not built for long time full throttle operations. Get those industrial engines for generators.

hey, how about a motorjet? it's less fuel-efficient, but maybe it could very well solve the power problem. his engine drives a compressor that feeds something that acts sort of like an afterburner. simple and powerful for a reasonable efficiency. any comments?

"Can I use THIS motorcycle engine in aircraft?" - The Wright brothers are turning in their graves...

You will need a fixed landing gear and a braced wing to keep your mass at your target of 40 kg. Expect a maximum L/D of not more than 10, maybe even 8.
You say that the flight mass will be 100 kg. The weight will be 980 N. 1/10 of that is 98 N. To arrive at a power from that will need a few more heroic assumptions. Your wing loading is probably 12 to 15 kg/m² and your lift coefficient around 1.0. This equates to a flight speed of 15.5 m/s (= 56 km/h). I assume a propeller efficiency of 70%, so your power for straight flight will be 2.2 kW.
For decent take-off performance, add 1.5 m/s climb capability at an air density of 1.1 kg/m³. The extra power to climb with those 1.5 m/s at 15.5 m/s and 70% efficiency is another 2.2 kW.
The sum is 4.4 kW which is 5.9 HP. Running the engine at this power level means 54% of maximum power. Make sure you have a gearbox which reduces prop RPM to not more than 2000 in order to make those 70% realistic. The wing will need to have about 8 m² area and the tail another 1.6 m². Will that all fit within your 40 kg?
I expect that 80 kg is a more realistic empty mass. Now the wing will measure 11 m² and the power demand climbs to 3 kW for level flight and an extra 3 kW for climb. Total power will be 8 HP or 75% of maximum power. Ask yourself how long the engine does this and how big your radiator needs to be in order to avoid overheating.

The extra power to climb, with a mass having a weight of 980 N, at a vertical speed of 1.5 m/s with an efficiency of 70%, is 980 · 1.5/0.7 = 2100 watt, not 4.4 kW...

No, I didn't. It's plain physics... You are in level flight and wish to increase your altitude. The energy you need for that increment is the difference between the gravitational potential energy of those two flight levels, and the power needed is energy/time...

@xxavier: I used the total power, not the increment for climbing. Sorry!

This has been done, but here are some things you'll need to consider.
Air-cooled motorcycle engines are designed to produce short bursts of peak power, and to be as light as possible. Lightness is an advantage for any airplane engine, but having an engine designed for brief bursts of high power is not. An aircraft engine has to operate for long periods at high power settings without overheating; if you try this with a motorcycle engine it is likely that you'll burn the exhaust valve, warp the cylinder head and score the cylinder walls- in other words, wreck the engine.
In addition, to produce peak power in a small package requires that the motorcycle engine spin very fast- 6000 to 8000RPM- and a gearbox or speed reducer of some sort will be needed so you do not drive the propeller tips supersonic.
You might be able to use the gearbox built into the engine as a speed reducer, but it adds weight.
My recommendation is to find an industrial engine of about 20HP continuous rating at 3600RPM and operate it through a 2:1 reduction, derated to 15 or 18HP to reduce stress and increase life.


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How much aerodynamic heating do jetliners endure?
At cruise altitude (~ 33,000 ft) and cruise airspeed (~ 500 mph) how much do present-day airliners heat up due to air friction during their journey? Probably not much because there's still ice on the window outside when you're at cruise altitude, but by how many degrees is the plane's hull warmer than in case the plane stood still at that altitude (therefore, compared to the minus 56°C at the tropopause)?
I'm also curious about the Concorde which flew up to 60,000 ft high and at more than 1300 mph. To the air friction there also comes the heating due to supersonic flight, so all in all how much warmer did the Concorde get during its supersonic cruise?

Based on the formula already shown in this answer, we can calculate the TAT (total air temperature), which is the temperature reached at the stagnation point1, from the SAT (static air temperature) and Mach number:
$$ \frac{\text{TAT}}{\text{SAT}} = 1 + \frac{\gamma - 1}{2} M^2 = 1 + \frac{1}{5} M^2 $$
The resulting plot of TAT vs. Mach number at the SAT of -56.5°C (ISA tropopause) is shown below:

As you can see, a subsonic airliner (I assumed Mach 0.85) will still experience temperatures below freezing. Concorde however was reaching temperatures of around 120°C at cruise speed of Mach 2.02. This was already close to the limit:

Maximum Total Temperature (TMO):  127 Degrees Celsius (on nose)

(Concorde Performance)

As requested in the comments, here is a larger speed range including the SR-71:

The temperature could reach almost 400°C at Mach 3.2 (What is the true top speed of the SR-71?), but note that the SR-71 used fuel to cool the skin (the windscreen was the hottest part at 316°C according to Wikipedia.

1The TAT is measured by the aircraft with a probe that brings the air to rest w.r.t. to the aircraft (stagnation). The kinetic energy of the air is therefore converted into thermal energy, raising the temperature. The surface temperature of the aircraft at points where no stagnation occurs will be at a lower temperature. Thanks to Peter Kämpf for pointing this out.

Shouldn't the SR-71 be included as a data point, just because Cool!?

@FreeMan Actually even more examples would be cool to mention on this diagram, such as the X-planes.

Thank you for adding the Blackbird.

That's a very large temperature rise, I'm honestly surprised. Why don't we do something with it?

@Abdullah Do what? Drive a Peltier element? That adds weight and isn't particularly efficient. Besides, the temperature shown is only reached at the stagnation point.

Please add that the temperature shown is only reached at the stagnation point. Most of the surface experiences much lower temperatures and those come from friction heat, not stagnation.

@PeterKämpf Thanks, I added some explanation at the bottom.

The temperature change is relatively minor, but total air temperature is always much less than the static air temperature in flight, due to surface friction.
I have been able to increase fuel temperature in outboard tanks, when fuel recirculation wasn't enough to increase the fuel temp at northern latitudes, by increasing cruise speed by a small margin.  It's interesting to see the results.  The change isn't rapid or great, but enough to keep the fuel temperature off the lower limit (where gelling occurs), in certain cases.
Typically, TAT (total air temperature) will be about 30 degrees C warmer, than the static outside air temperature (SAT), for typical transport category aircraft (not supersonic), operating at the base of the transonic range (.80 M1 or higher).

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Who was the air traffic controller voice?
I recall reading somewhere that the 'Air traffic controller' voice (you've heard it on TV on many occasions) started with one specific person and his mannerisms have been copied by many air traffic controllers since, particularly in the US.  The article mentioned his name but I can't find the article and can't remember his name.
What was the name of this chap?
Edit: I think that I read this in Fate Is The Hunter, an autobiography of a pilot working in the '30s to '60s.  Unfortunately I don't still have my copy so I can't look it up.

There may be a specific actor who has a familiar voice and has played ATC roles, but in the real world controllers all speak with their own voice, they don't try to imitate anyone.  Any similarity comes from common phraseology, and the fact that they speak in a clear and professional manner.

The article wasn't referring to actors playing ATCs on television.  IIRC it was a cold-war era air force ATC that influenced peoples' mannerisms in that era, which in turn influenced the portrayal in the media.  I think that I saw this mentioned in Fate Is The Hunter, which is an autobiography of a pilot who worked in that era.

OK, your "you've heard it on TV on many occasions" got me thinking that way.  Actually, most often I find film and TV portrayals of ATC and Pilot communication to be not very good...

I'm sure I can come across a copy of "Fate is the Hunter" in the next few days.  That is an Ernest Gann classic.  I'll get back to you later, if someone else doesn't chime in with an answer or comment regarding this book first.

@quiet flyer Thank you but don't go to a lot of trouble on my account.  Unless, of course I've prompted you to re-read it (and it is actually quite a good book).

In his famous "SR-71 speed check" story Brian Shul references what he calls the "Houston Center" voice, referring not to the FAA's ZHU facility but NASA Mission Control at the Johnson Space Center where the Apollo mission controllers used the callsign "Houston."

You are thinking of a reference to Charles ("Chuck") Yeager.  He was certainly not an air traffic controller, but there has definitely been a statement published in print saying that his manner of speaking over the radio has been the model that has been copied numerous other times (in movies and by other pilots in real life, as I recall the statement saying.)
The reference was in the book "The Right Stuff"-- pages 33-34 and possibly a few more (not shown) in this ebook edition.

I have read Space but I'm pretty sure I've never read The Right Stuff, and I'm pretty sure the reference I did read was talking about an air traffic controller.  Unfortunately I lent my copy of Fate Is The Hunter to somebody and never got it back so I can't check that.

Well, that's all I've got.  See updated version of answer and check the reference-- if that's not it, I'll be interested to see what the actual best answer turns out to be.

Thanks for the reference anyway.  Although I'm pretty sure it's not what I'm looking after, it's an interesting piece of history.

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How does induction ice form in and affect jet engines?
Recently someone talked to me about the possibility of induction ice on the engine:
He told me that because of conditions around fan and compressor blades (mainly because of pressure and temperature) some ice may form, under certain exterior conditions, and destruct the engine by ingestion or by stall of boundary layer.
My question: It is really possible? If yes, can someone explain to me how it works?
I don't find proper explanation on Google.
I raised this question after this hypothesis:

Icing inside jet engines can become a hazard at high altitude near large thunderstorms, when moisture turns into tiny ice crystals that can be sucked inside an engine. If the ice melts but then freezes again on metal surfaces, accumulating ice can either break into chunks that damage turbine blades, or melt and douse the ignition system.

source

Induction ice can be a problem for jet engines. Other answers have explained how the ice forms. The air expands and cools in the inlet before entering the engine. The first stages of the compressor do not compress the air enough yet to warm it enough to prevent ice from forming. Low power settings can also make it easier for ice to form. Certain atmospheric conditions make ice more likely to form.
The FAA released AC 20-147, which "provides guidance and acceptable methods, but not the only methods, for demonstrating compliance with the applicable engine induction system icing and engine ice ingestion requirements." This describes different types of icing, and the effects it can have on engines. It also describes how engines should be tested to conform to the regulations for these conditions. Ice can form in various places:

The fan or compressor blades themselves
Vanes
Sensors
Bleed ducts

This ice can cause various problems:

The ice can come off and damage engine components downstream
The ice will reduce the effectiveness of blades, making it harder for the engine to compress the air
The ice can impede the flow of air in the engine
The ice can block sensors, preventing the control system from correctly monitoring the engine, which can lead to power loss or instability

One form of icing that has been an issue lately is ice crystal icing. This is caused by small ice crystals in the upper atmosphere, particularly above convective clouds. The FAA has issued an AD (Airworthiness Directive) recently about this.

This type of icing does not appear on radar due to its low reflectivity, and neither the airplane ice detector nor visual indications reliably indicate the presence of icing conditions. Therefore, it is often undetected by the flight crew. Flight in these conditions can cause ice crystals to accumulate in the core gas flow path of the engine.

There were previous events on GE CF6-80 engines (used on 747, 767, A310, A330, and others) that the FAA looked into.

In the events leading to those prior ADs, the ice has shed during throttle transients and in the descent phase of flight, causing temporary thrust loss.

However, there have been incidents with the newer GEnx engines on 747-8 and 787 aircraft which have been more serious.

The new events that prompted this AD, however, have occurred during the cruise phase of flight and caused permanent damage (beyond maintenance manual limits) to the engine compressor. In all thrust loss events, data indicate that ice crystals entered and collected in the initial stages of the compressor. Engine temperature data indicate small ice accretions were shed through the core of the engine.

The engine manufacturers are working to resolve the issues on the aircraft currently affected, and in the meantime flight crews are advised to avoid weather conditions where ice crystals tend to form.

More definitively, the AD includes immediate mitigating procedures for the affected aircraft which prohibit flight within 50nm of amber and red radar returns that are displayed below the aircraft’s flight path during operations at or above 30,000 ft, when approaching, or in, IMC or visible moisture.
Note: Boeing has also published an updated Flight Crew Operations Manual Bulletin with specific operating procedures for flight crews.

Nothing at all against Jon Story's nicely technical answer, but I think this one is a far better option with the references to ADs indicating potential issues due to icing in engines.

Most of this refers to the problems of pre-existing external ice being ingested, which wasn't really the question... it doesn't refer to the way in which ice forms in the engine, nor the level of risk inherent in that. There's a big difference between an engine ingesting 10kg of tiny ice crystals a second, compared to the minute particles which will be created by the fan pressure changes.

As per the FAA's AC 00-6A Chapter 10:

The ... compressor in a
  jet engine forms a partial vacuum in the intake. Adiabatic expansion
  in the partial vacuum cools the air. Ice forms when the temperature
  drops below freezing and sufficient moisture is present for
  sublimation....Induction icing potential varies greatly among
  different aircraft and occurs under a wide range of meteorological
  conditions. It is primarily an engineering and operating problem
  rather than meteorological.

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How do SIGWX chart updates work?
AWS says the SIGWX charts are issued at 0000, 0600, 1200, and 1800 UTC, which I assume matches the VT times on the charts themselves. What confuses me is AWS' parenthesized statement based on the current ETA model available (http://aviationweather.gov/products/swl/info).
Let's say it's 1100Z on Monday morning, and I'm trying to use the SIGWX charts to predict weather as far in the future as possible using SIGWX charts available to me at this time (again 1100Z on Monday morning), and only SIGWX charts. I know there are other information sources, but this question pertains only to SIGWX charts and their timing.
1) Which SIGWX charts are relevant for future weather developments (at 1100Z Monday)? The only two updates scheduled at 1200 and 1800 at this time?
2) I'm assuming that today's (Monday's) 0000Z and 0600Z SIGWX charts will be updated using the current ETA model available at the time the 1200Z chart is published. So is it fair to say that "all" SIGWX charts are updated when a SIGWX chart is published (at 0000, 0600, 1200 and 1800)?
Thanks for any clarification.

First, an explanation of "the ETA model"
The science of weather prediction is the science of data analysis.  Meteorologists receive data from weather reporting stations worldwide, and from that data, they attempt to predict the future, using the laws of physics as well as assumptions and educated guesses.
Educated guesses?!  Say what?!
That's right.  Weather is one of the original chaotic systems, and although the weather obeys physical laws, we can't gather enough information about the current state of the atmosphere to predict what will happen in the long term with any degree of accuracy.
Bummer.  So what are all the computers for?
Well, we've found that supercomputers can predict the weather out to a few days ahead with a reasonable degree of accuracy, even without perfect information.  They do this by applying educated guesses and rules of thumb.  Using different assumptions gives a different forecast model.  Some of the more well-known models are the GFS, the NAM, and the EMCWF.  The ETA is one of the lesser-used models.
Great.  So what does "based on the current ETA model" mean?
Simple.  It means that the SIGWX chart gets created based on the most recent data that the ETA supercomputer churned out.  If the chart is supposed to come out at 1800 UTC, but the ETA model run doesn't complete until 1845, then that new forecast data doesn't make it into that chart.
Enough.  Just answer my questions.
Okay.

which SIGWX charts are relevant for future weather developments (at
1100Z Monday)?

All the SIGWX charts are relevant, but the newer charts will have the newer forecast data.  That doesn't necessarily mean that they're going to be more accurate, but they probably will be.
To use your example, it's 1100Z on Monday morning.  The 0000 chart says "IFR" but the 0600 chart says "No IFR".  There might still be IFR out your window - nothing says that the supercomputer will get it right.  Maybe the older one was more correct than the newer one.  Doesn't happen often, but is it possible?  Sure.

So is it fair to say that "all" SIGWX charts are updated when a SIGWX
chart is published (at 0000, 0600, 1200 and 1800)?

No, as soon as a chart comes out, it becomes part of history.  Charts are not updated with new weather information; they remain as a historical record of what was predicted to occur, given the best information possible at the time (which, for the forecasters, can be just as valuable if not more valuable, to see where they went wrong in the past).

Thank you for the thorough answer. I will upvote it as soon as I can.

Just a historical note here.  The ETA model was retired in 2006 in favor of the NAM model (WRF-NMM).   These days with the wealth of model data available (GFS, NAM, RAP, HRRR, SREF and MREF ensembles, etc)  I'd be surprised if they ingest solely the NAM for generating SIGWX charts.

Not only thorough and useful, but entertaining, as well!

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How common are 400Hz supplies in aircraft?
I understand that 400Hz, 26Vrms power is found in a lot of aviation equipment. But how common is it? Is it in every plane and every instrument or is it restricted to certain sectors, like military, commercial, or private planes?
Is it used a lot in ground applications to power aviation equipment, like in a workshop/lab setting?
The reason I ask is that I'm trying to gauge interest in a small 400Hz, 26Vrms power supply for the benchtop (plug it into regular 60Hz 120V power). From what I can see, these are hard to find below a few thousand dollars.

In modern military and commercial aviation, aircraft power is 3-phase 115V at 400Hz (AC), and 28V DC.  This is standard just about everywhere.
AC power is 3-phase 115V AC at 400Hz (alternating current).  When the engines are spinning, this power is produced by onboard generators (alternators) feeding redundant power buses.  On the ground, this is typically sourced from a ground power unit (GPU).  During emergencies, AC power may be produced by an auxiliary power unit (APU).
DC power is provided by converting a portion of the 115V AC power to 28 volts DC for powering many of the avionics systems and charging the aircraft battery.
EDIT: My apologies, contrary to my original answer (I was thinking powering the cockpit displays/indicators mostly), a lot of navigation and pitch/attitude sensors take 26vac/400Hz input power.  Rate gyros, compass, flux valve, etc.  See, e.g. this section from a Cessna Citation.

The link is 404 Not Found

One example of 26VAC in aircraft is in old analog avionics (think DC10-era autopilot or navigation instruments), where it is used as a reference voltage for the signals transmitted using synchro. I think these days it is fairly uncommon.


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How are performance variables accounted for when testing solar-powered aircraft?
With so many factors that could affect performance (wind, visibility range (amount of light), angle of the sun, etc.), how do they accurately test solar aircraft flight time? Any tips for doing my own experiment for a small solar RC plane (vs non-solar of same type)?
Thanks for any help!

Well, first you have to define what you mean by "testing efficiency" - what criteria matter to you? Flight time on a single charge/tank of fuel? Recharge/Refuel time? Cost? Speed? All of these are things that have trade-offs, so you need to decide what each is worth, and then you can design experiments to test them...

just flight-time for a single charge (ohers may come later)

Usually when test results are potentially affected by many factors, standard test conditions are defined. When tests are performed under these standard conditions, the results can be compared between products.
Since these standard conditions rarely exists in nature, testing under standard conditions is not possible. Instead, a test programme is developed that tests under a number of non standard conditions to identify the effect of individual factors and correct for that. In addition, components of the system under test can often be tested in an isolated controlled environment. In this way, standard conditions can be achieved for individual components and the effect of deviations for the standard conditions can be determined as well.
For the example of a solar aircraft, the efficiency of the solar panels could be determined independent of the aircraft. The same holds for the battery, the engine, the propeller and other components that the propulsion system may depend on. Not only the performance under standard conditions can be determined, but also under non-standard conditions.
The power requirements of the aircraft under a range of conditions can be determined by test flights. This could involve horizontal cruise flight segments at various speeds, altitudes and temperatures, and also gliding tests.
The maximum achievable flight time under standard and non-standard conditions can then be calculated from the results of all the acquired test results, without ever having flown under these standard conditions.
For validation of the methodology used, a test flight can be done do determine the actual flight time under non-standard conditions. When the results of the test flight agree with the calculated flight time under the same non-standard conditions, one can assume that the methodology is correct.

For you own RC plane such a test programme is probably out of scope. A simple way to gather some data is to perform two flights with the same aircraft. Both time start with a full charge (I assume there is a battery involved). Test how long the flight last with the solar panels. For the second flight, disconnect the panels and see how long the flight last then. You now know how much the solar panels contribute in terms of power production. Then fly with the non-solar plane and see how long that lasts. It will probably last longer than the solar plane with the panels disconnected because it doesn't have to carry the extra weight of the panels. This way you learn what the addition of solar panels costed in terms of flight efficiency.

For a small RC craft, I'd expect flying with just the fully-charged battery will actually be more efficient than a fully-charged battery + solar panel. The extra induced drag + parasitic drag is probably worse than not having the panels at all. It usually takes rather large aircraft operating with next to zero payload and at very low speeds for solar-powered flight to achieve more mileage or flying time than the same aircraft sans the panels. The ones I've seen have the wing span of an airliner and can barely lift the pilot. Solar power density isn't as much as many seem to think it is.

For large solar aircraft, it is advantageous to be flying above the clouds, for predictable and uninterrupted solar performance.
There are flight test areas (airfields with a large-enough flight range) where meteorological conditions and test availability allows companies to fly several sorties and calibrate their flight-performance parameters. Spain, Sweden, Canada and USA has these sites. The FAA has a list of US test sites on the web.
Solar-powered aircraft have constant weight and CG which makes the tests much simpler than other fuel-powered aircaft.
For personal flight tests, flying a circular or figure-8 pattern at a constant altitude, with as little turbulence as possible, for a full charge, would reveal the necessary information for flight times. But if the flight takes longer than half an hour, it would be best to have the autopilot fly. So that you could get the piloting efforts (and errors) out of the equation.
Of course, time of the day and day of the year are important parameters but you can calculate their effects on your solar panels by hand.

Are you sure you meant to say that flying above the clouds is a reason for predictable performance?

thank you @raptortech97, corrected the sentence, it was a misexplanation. hope it's better now.

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Can single engine aircraft fly in class A airspace?
A friend surprised me the other day by saying that single engined aircraft are not allowed to fly in class A airspace.
I understand that the aircraft is under IFR in class A, with ATC clearance, (flight plan) and must meet PBN specifications, however I can’t find any documentation in my textbooks, EASA or ICAO stating that single engined aircraft are not allowed to fly in class A.
If anyone could resolve this issue it would be much appreciated.

This isn't true.

Do you mean that SE can fly in class A or SE can’t fly in class A.

I mean they can fly, there is no requirement for multiple engines in Class A airspace (or any engines at all). For example the Cessna P210N has a service ceiling of 27,000 feet and owners regularly cruise in Class A airspace.

In fact, zero-engine aircraft can and do legally fly in Class A airspace, as long as they have the appropriate waiver or IFR clearance

Indeed, there are at least two single-engine airplane operators (PC-12, C208) that have scheduled flights to KATL. I have noticed that they will take a different departure path than that used by the jets, though, presumably to get out of the way of faster traffic.

Are you asking about EASA regulations?

Single-Engine aircraft can fly in Class A airspace. Pressurization or oxygen masks are required above 18,000 feet (no cannulas).

That requirement is not listed in Part 91 or in Part 121 but is noted in the FAA document linked in the post by Ron Beyer. Do you know which regulation is the source of the requirement?

I thought it was a bit odd to reference the 18000ft thing, then I realised that class A starts at higher altitudes in the states. Not like the U.K. where some class A airspace starts at altitude 2,500ft!

@JScarry, it is 23.1447(c).

@Dave-CFII Part 23 starts at §23.1457, so there is no §23.1447. There are a bunch of rules in Part 25 regarding oxygen, but those apply to transport category planes and don’t mention masks specifically, but it may be implied by the pressure requirements.

I guess you got me. It used to be in 23.1447, but I guess they've up and changed the entire part.

Here is what the FAA says about operations in Class A airspace:

Must conduct operations under instrument flight rules (unless otherwise authorized)
Must obtain ATC clearance before entering the airspace
Unless authorized by ATC, you must have:

Transponder with altitude reporting
ADS-B (as of January 1, 2020)

Oxygen required if not pressurized

Crew at all times above 14,000 feet
Passengers at all times above 15,000 feet
Cannulas can be used at altitudes less than 18,000 feet
Oral/nasal rebreathers can be used up to 25,000 feet

There is no mention about aircraft equipment (other than being on an IFR flight plan, which means you need to have an aircraft with IFR instruments). Single engine aircraft like the Cessna P210, the Pilatus PC-12, the Piper M350... the list goes on, all designed to operate at high altitudes with a single engine. The Cirrus SF-50 cruises at 28,000 feet on a single turbojet engine.
It may be that your friend was thinking about ETOPS operations required of commercial aircraft, those require at least 2 engines for certain routes over water or remote areas.

How about EASA or UK CAA

@Ollie As far as I know, EASA and CAA are the same requirements, the only difference is in the UK, some airport terminal control areas are also class A, like around Heathrow. Also around the Channel Island Zone, Class-A starts at 8000 feet.

By my reading, you've actually reworded 91.135 to mean something different than what it says.  The first sentence: "Except as provided in paragraph (d) of this section" and the first sentence of paragraph (d) "An operator may deviate from any provision of this section under the provisions of an ATC authorization issued by the ATC facility having jurisdiction of the airspace concerned." means that your first bullet (must conduct under IFR) is not always correct.  An example would be a glider operation using a wave window that allows entry into Class A.

@bclarkreston I added clarification, ATC can clear you into Class A without IFR, but typically you need to be on an IFR flight plan. There are exceptions that ATC can handle (this is the case with pretty much all controlled airspace).

@RonBeyer yup...that's better...thanks!

@RonBeyer it’s not just around Heathrow where class A starts at 2500ft. It’s such a busy airspace that class A starts at ~4500ft to FL70 across most of central England.

@Ollie I said "airport terminal control areas, like around Heathrow"... It shouldn't be construed that I meant "only around Heathrow". Central England is pretty busy airspace wise, you have London, London City, Gatwick, Birmingham, Norwich, (and a little further north) Doncaster, Manchester, etc. Not saying those are all Class-A, but yes, it is very busy throughout the central area.


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Why are gate numbers also marked on the outside of a terminal?
As per this question.
What are these numbers visible from the runway at Narita airport?
A UK document is linked in the accepted answer  but it doesn't list big number marking on the outside of the terminal. Surely such numbering is of little value to pilots- too coarse of location information. So who use such a gate number and how?

Why do you assume it's too coarse for pilots?

How much finer would you like the gate number to be?

You can line up a plane to some number marked on the terminal raised to the height the building over 300 metres away in low light. If every pilot can do that why have any other markings. A mark at the far end of the runway should be enough for them land on that runway.

See this discussion at airliners.net - "how do pilots find their way through the taxiway labyrinth to the exact gate where they should park?" -  "Gates are also marked with their numbers, either on the terminal building itself, the jetway, or in some places, painted on the ramp." - The numbers on the building are each at the precise position of each gate.

The signs on top of the jet bridges are useful in identifying your assigned gate. Without these signs it would be easy to misidentify a neighboring gate if it also had a ground crew ready to accept an airplane. They are also useful for keeping an eye on an occupied gate from a distance while you wait for an aircraft to leave. They are also illuminated which helps locating the right gate at night or in low light.
The electronic signs high on the terminal building are helpful to aircrew and ground crews. They often list aircraft info, departure/arrival airports and time to scheduled pushback.
The on ground painted lines are useful for initially lining up for a gate and in making the elaborate maneuvers required getting into certain tightly spaced gates. Once you get this close you are being guided by ground crew and have wing walkers watching your clearances. The final painted ground positions for nosewheel position (by aircraft type) are only useful to the ground staff guy with the wands -- you can't see them yourself when needed.

Of course it's helpful to the pilots, and the ground crew, too. Even if you know the airport layout in detail, it's easier to go to the gate signposted as C27, say, than to go to the seventh gate on the left. Now remember that everybody who has ever flown a plane into that airport has flown there for a first time, and everybody who's ever worked ground crew there had a first week.
Just imagine the corresponding situation of a car park with somewhere between a few tens and a couple of hundred bays, where it's very important that you park in your assigned bay. Would each bay's number be written on it? Of course it would.

Not with the location of these signs- you aren't considering the location near the roof level of the terminal. Targeting  A marker on a ramp would be much easier to spot and position yourself.

Putting the signs at height makes them more visible from more places. That's why they put billboards up on walls instead of painting ads on the ground!

No one needs to line with a billboard- you'd want to be at right angles from a gate location- right?

@user2617804 the numbering helps you locate the right gate. Ground markings and other visual aids help you park at the gate.

@user2617804 As a person who made everyday use of large gate number signs mounted high, I can assure you that in general the higher and the bigger they are the better. You may be having to locate them from a quarter of a mile away. Also, the entry to and parking at a gate is not always at right angles, especially at concourses that have an expanded circular portion at the end.

Gate designation that are visible to the outside are important for a number of reasons.  Mainly for Ground Support teams that service aircraft, fuel the planes, mechanics, baggage, and yes even pilots to use to find the correct gate to park upon arrival.

What does this add to the existing answers?


text:
How is a turbine engine rotated for starting and inspections?
I'm doing an assignment for my Level 3 Aeronautical Engineering course and stumbled on this question and I can't seem to answer it (we have had no teacher for 4 months and therefore we have to learn by ourselves):

Name and explain the type of coupling between the main engine rotating
assembly and the accessory gearbox and therefore how a gas turbine
engine is rotated for:
a) engine starting
b) internal inspection (boroscope)

Any help is greatly appreciated.

On the CFM56-7B engine powering the Boeing 737 NG family, the starter and the handcranking input are driven by the accessory gearbox which role is to link some components to the high-pressure shaft (N2 rotation speed). This gearbox is located outside of the engine core, on the fan case.
Accessory gearbox on the CFM56-7B

Picture: CFM56-7 exploded view. Source.
Corresponding schematic view:

Picture: CFM56-7B AGB, horizontal and radial shaft, TGB, IGB and engine shafts. Taken from this answer.
AGB to core shaft
There is a gear train within the AGB with its input gear used to transmit movement between the AGB and the engine core.
The AGB is connected to the N2 shaft by two bevel gear gearboxes:

The transfer gearbox (TGB) fixed just aft of the AGB, on the fan frame case. A horizontal shaft links the AGB input gear and the TGB. A radial shaft is also connected to the TGB.

The inlet gearbox (IGB) fixed close to the No 3 bearing, in the fan frame hub. One bevel gear is coaxial to the HPC shaft, the other is coaxial to the radial drive shaft.

The radial drive shaft runs within the 9 o'clock strut of the fan frame.
The described mechanism is bidirectional, the AGB is normally driven by the HPT shaft, but when using the starter or the handcranking mechanism, the pads rotate the AGB input gear using the gear train, and the input gear then drives the intermediate shafts and gearboxes.
Handcranking drive pad and starter
On the picture below,

The black handcranking drive pad is visible on top (cover removed). It is used to turn the N2 shaft during borescope inspection (the N1 shaft can be rotated by moving the fan blades manually).

The starter is the large assembly with the yellow cover protecting the large pneumatic inlet (air is used to turn the starter).

Picture: CFM56-7: Handcranking pad and starter, source Youtube.
During borescope inspection, either a pneumatic motor or a square drive socket is used:

Picture: Handcranking drive pad and starter. Source: Youtube.
Alternatively on other CFM56 engines the core can be turned directly from a pad on the TGB.
Accessory pads on the AGB
The accessories have allocated pads on the AGB:

Picture: Accessories and their allocated drive pads on the AGB. Source CFM56-7B manual.

Also: Actual photos of the gearboxes in this answer.

Many engines do not have externally accessible gearboxes, and boroscopic exams are frequently done with the engine in situ. While your image is from a recognized text, it does not reflect the diversity of configurations, and can be misleading.

This answer gives a pretty good idea how some engines turn internally, via gears.

Used in Pakistan International Airlines #8303 update (from 05 min 50 secs. Published 2020-05-23). Channel blancolirio by Juan Browne. It is acknowledged at 07 min 59 secs. Pakistan International Airlines Flight 8303.

@PeterMortensen: Thanks for telling, I appreciate. On the CFM56-5B powering the A320 family the AGB is at 6 o'clock and the radial drive shaft is vertical, but otherwise the design is very similar (on this picture a 7B at the foreground, a 5B at the background).

Having done some engine boroscopic inspections in the past, usually a pneumatic drive motor of some type is used to rotate the inner workings of the engine under inspection. On the General Electric CF6-6 and CF6-50 series of turbine engines, there is a drive pad at the 6 o'clock position where the tower shaft that drives the accessory gearbox is located. The cover plate is removed and the pneumatic drive motor (which is usually included in the borescope kit) is hooked up, and the compressor and turbine components can be rotated during the inspection. On smaller turbine engines, usually the inner workings are rotated by hand, using an old-fashioned socket wrench and an extension bar.

Does this mean that on the CF6 the accessory drive has to be exposed to index the shaft?

Engine starting: Normally by a starter generator, but some engines are air start.
Borescope inspection: It varies quite a bit.  Some have tooling on the shaft which can lock the shaft, and I have seen others without any way of holding the shaft for an in situ inspection.  I this case, there are normally two mechanics working...one holding, and the other inspecting.  If the engine is removed from the aircraft, then there are clamping points, but I can say that I have seen engines without any, and improvised clamping by the mechanic. I do not know but there may be some manufacturer's guidance on clamping, because obviously one could clamp in the wrong spot.
After my last suspected FOD on a turboprop (PT6), two company mechanics did a borescope inspection. One held / manipulated the prop, while the other threaded the borescope.


text:
What does "redatum" means?
I was doing a question about a fully hydraulic elevator and it's position in relation to the trimmable horizontal stabiliser and the explanation of the answer contained the word "redatum". What does it mean?
Full question, answer and explanation:

Q: What is the position of the elevator in relation to the trimmable horizontal stabiliser of an aeroplane with fully hydraulically operated flight controls that is in trim?
A: Elevator deflection is zero
A fully hydraulically operated flight controls is an irreversible system, and the trim is a redatum, as there is no physical connection between the stick and control. Therefore there will be no elevator deflection when trimmed.

My first reaction is that it's a typo since "redatum" is not in any dictionaries I have. Searching the interwebs I find that it is a commonly used word in geology papers. The papers are all super technical and I know nothing about geology, so it's a bit tough to discern the meaning. But what I can gather the word is used in data analysis of seismic or radar returns. If you don't get an answer hear you might try at English.se

I'm pretty sure that has nothing to do with this case, but thank you very much

I think in the geology papers it is being used similarly to what Koyovis says. It seems to be a way of removing aberrations from data by setting an artificial reference plane against which sensor output data can be analyzed. Or something like that. All the sources I find on the web are from technical geology papers. So what Koyovis says makes sense

You must have accidentallly asked your elevator question on a geology website :P

I've never heard of the term, but from the context of the explanation it appears that they refer to a situation where the elevator neutral position is relative to the stabiliser, not to the aircraft. So stick moves elevator relative to aircraft and stabiliser, trim moves elevator relative to aircraft only because the elevator neutral position follows the stab position. Like the stabiliser/elevator arrangement on the B737.
Image source

That's what I thought too, and it agrees with the anser as well. Thank you!


text:
Are there countries where not all plane crashes are investigated (even if deadly)?
background information
I was struck by the news of this deadly plane crash on 20 Dec 2018 which claimed 8 lives and apparently is not investigated as the plane and its potential survivors were not even searched for:

A Gomair Antonov AN-26 on behalf Central Electoral National
Independent Commission (CENI), registration 9S-AGB performing a cargo
flight from Tshikapa to Kinshasa Ndjili (DR Congo) with 7 or 8 crew
and election materials, was cleared to descend to 5000 feet on
approach to Ndjili's runway 06 when the aircraft went missing about
20nm short of Kinshasa's runway 06 at about 10:00L (09:00Z). The
aircraft was found about 24 hours later in hilly terrain (peaks rising
up to 700 meters/2300 feet MSL) about 19nm before runway 06. Local
Congolese media report Authorities did not launch a search for the
aircraft. The aircraft was found by random chance by a local, who
reported 8 bodies in the wreckage. (source)

question
Aren't countries bound at least (if nothing else) by international treaties and ICAO regulations to

perform SAR operations for missing aircraft lost in their sovereign airspace
and investigate the cause of the accident ?

In which countries can deadly plane crashes lack either one or both ?

scope
Q (from a comment below): "There are countries which may not have the resources to conduct their own investigation of a crash and may request assistance from another country."
A: Good point. Countries unequipped to handle S&R or crash investigations can voluntarily outsource either one or both to other countries. For the pupose of this question, a crash in country X investigated by country X or Y does equally count as being investigated, so this is NOT what my question is about. I'm asking about dead passengers searched for by NOBODY and crashes investigated by NOBODY.

Well yeah, in countries with non-robust civil societies you might say, this kind of stuff may only get investigated if the country of the manufacturer gets involved, because its civil aviation authority may not be equipped.

Your question is a bit vague. Investigated by whom? In the US, military aircraft accidents are not investigated by the NTSB, for example. There are countries which may not have the resources to conduct their own investigation of a crash and may request assistance from another country.

Congo is a defunct state. It's security forces don't have resources or coordination to mount an effective search and rescue operation at this time.

@JanHudec: if unequipped, they could outsource / ask for assistance (see Juan Jimenez's comment)

re: "Investigated by whom?" By anyone. In this case apparently nobody looked for the plane. – re: "countries requesting assitance" good point! I'll clarify my question accordingly.

@summerrain, I suspect you are underestimating the fix that Democratic Republic Congo is in. Search, rescue and investigation require that at least somebody controls the territory and can ensure safety of the rescuers and investigators. From what I read about DR Congo, the official security forces likely don't and the situation is so messy that nobody else does either. Also the flight being related to the two years overdue elections, I wouldn't be too surprised if it was a sabotage and at least some officials actively opposed such investigation.

@JanHudec: I don't underestimate but actually imagine this to be the situation in many war-torn countries – hence this question.

@summerrain, yes, it probably is. The problem with war-torn countries is that the situation changes quickly, so at one time investigation might be possible and at another not.

Who is the DRC going to "voluntarily outsource" the investigation to? Is there a crew of experienced investigators who will come work there for free (keeping in mind that the country has significant security and public health concerns)? Will they do so for every incident?

Investigators are not freelancing but employed and obviously get paid for by their national employer, even on missions abroad.

@ZachLipton even worse, who'd risk their life in a warzone investigating the crash of an aircraft without very good reason? The Dutch civil aviation authorities did in Ukraine after the MH17 shootdown and had major problems, and they were there on invitation of the Ukrainian government...

As of 2017 191 of the 193 countries are signed to the ICAO anex and there are 122 national aviation authorities so at least some countries lack serious formal aviation authority and often look to other nations for help. Smaller, war torn nations or nations that do not have the facilities of first world countries may simply not have any ability to provide SAR operations. Its not just about having a helicopter or two hanging around but having a trained crew, a flight doctor and a hospital to bring the potential survivors back to.
Countries are under no obligation to provide SAR operations and generally speaking international treaties can't really force any country to offer something like that. In some cases the crash sites can be difficult to access or outside the range of practical rescue at the time of the incident. In these cases there may be incidents where a country simply cant investigate as it may risk the lives of those investigating.
There are some notable exceptions, the FAA will provide aid to any investigation involving an American built aircraft or having Americans on board anywhere on the globe. This is of course assuming they are permitted to provide such aid (which may not always be the case). The FAA also has a lot of tooling for recovering black box data and in many cases foreign investigations will send the black box to the FAA facility if they lack the data reading tools.
Its not always known when or where planes crash. This generally applies to smaller aircraft that fly VFR from uncontrolled fields. Broadly speaking its possibly to not tell anyone you are going flying, depart at night, and crash in a remote area unknown to anyone that you were ever up or what your flight path was. In this case its possible no searching is done if no one even knows the aircraft needs to be located.

+1 great answer. re: "its possibly to not tell anyone you are going flying, depart at night, and crash in a remote area unknown to anyone that you were ever up or what your flight path was. In this case its possible no searching is done if no one even knows the aircraft needs to be located." Oh ok. I thought every flight needs to file a flight plan. (Among other things exactly for this purpose so that in case of a crash rescue teams have at least an approximate search location to go by.) Is this false ?

@summerrain all IFR flights need to file but VFR flights are under no requirement (here in the US) to file a flight plan.

@summerrain See this question, although it's for the US only.

@summerrain vfr, no flightplan filed typically. And of course in disfunctional countries, a filed plan may be lost. And aircraft get stolen.


text:
How long is the EASA ATPL theory exam valid?
I got the JAA CPL and IR and ATPL theory examination in Spain in 2009. They are all expired by now. I didn't renew them until now because I'm flying with the CAAC license and got 3600 hours experience time.
I want to endorse A320 type rating on the EASA license in order to fly in other countrys. They recruiment requirements are EASA CPL(or ATPL) with IR and MEP and (frozen ATPL).
My questions are:

Is the ATPL theory examination expired or is it called the frozen ATPL?
If I hold the valid IR issued by an ICAO member state China,  is the Cross-crediting applicable to RENEWAL of the EASA IR and MEP?

This question is better to be asked to the CAA were you want your EASA A320 rating to be endorsed by. They will be the ones that can clarify this to you.
I remember that in 2007 my JAA frozen ATPL had a valid theory for around 7 years.

FCL.025 is very clear on that and it says:

(2) The completion of the airline transport pilot licence (ATPL) theoretical knowledge examinations will remain valid for the issue of an ATPL for a period of 7 years from the last validity date of:
  (i) an IR entered in the licence; or
  (ii) in the case of helicopters, a helicopter’s type rating entered in that licence.

ATPL theory exams are valid for 7 years of the date of expiry of IR, entered in EASA licence. So it depends when your EASA IR expired. ICAO IR unfortunately does not count in this case.
If your ATPL theory exams are not valid anymore, you will have to do them again. It is possible to transfer A320 rating from CAAC (ICAO) licence to an EASA one - depending on your experience. You would need, however, to do LST (licence skill test) on A320 in the simulator and pass all 14 EASA ATPL exams again.


text:
When does an instrument approach "begin" per §135.225(a)
§135.225(a) states that, "...no pilot may begin an instrument approach procedure to an airport unless—
(1) That airport has a weather reporting facility operated by the U.S. National Weather Service, a source approved by U.S. National Weather Service, or a source approved by the Administrator..."
Assuming that a part 135 pilot is operating to an airport where the weather reporting is out of service, and the pilot is not conducting eligible on-demand in accordance with §135.225(b). What portion of an IAP (if any), can be flown in this case without violating the intent? Is the IAP considered to "begin" at the IAF, IF or, for the intent of this regulation, at the FAF? Or, does it begin at the point ATC says, "cleared for the approach?" In a case such as this, the 135 pilot is needing a descent to VFR weather, since an IFR arrival (including visual or contact approach) is not authorized. Being able to descend to a the initial or intermediate segment altitude would obviously be an advantage.
If a local (or designated facility) altimeter setting is not available, terrain clearance would not be guaranteed when descending to the initial and intermediate altitudes. In that case I believe that ATC could not give a clearance for descent to those altitudes.

You should be more specific in the title. If you question is "when does the approach described in far 135 begin?",  the title should be "when does the approach described in far 135 begin?"

Thanks, revised.

Being vectored is a totally independent phase of flight, generally considered en-route thus you are not on any part of the approach while being vectored.
However if you enter the approach at an IAF then that would be on the approach as your only lost comms/own-nav solution is then to fly the whole approach through the missed. So it seems reasonable that being both cleared for an approach and exiting the enroute environment (those bits which are not on a low alt. chart) would be entering the approach.

Beginning an instrument approach would mean being established on any segment of that approach, whether the initial, intermediate, or final approach segments, and they have received ATC clearance to do so.

That makes sense to me, though I would love to see a source reference. I've been unable to find any letter of interpretation or reference that defines "beginning" or "initiating" an approach. It's not unusual for a pilot to be cleared to fly the initial and intermediate segments of an approach, including descent to the segment minimum altitudes, without the phrase "cleared for the approach." Perhaps the legal point of "beginning" is at the point ATC says "cleared for the approach," and that could be at any point prior to the FAF.

I agree that flying any portion of the published approach means you have begun that approach.  Reference?  The published procedure, and the fact that you are flying the profile means you have begun.  I wouldn't play a semantics game with the word "cleared" either.  In the event of a mishap or other incident would you feel comfortable defending your decision to fly a published approach procedure without weather because the controller said you could "continue", but didn't actually clear you?  Better have a good lawyer if a passenger is suing...

Well both items are required to fly an instrument approach, Michael

I have been inside of an initial fix plenty of times and been told to continue and expect clearance. i.e. “XXX, continue on the localizer, expect ILS clearance in 1 mile.”  Either way, the point made is that if you are flying any segment of the approach you have begun that approach. Spoken words do not alter that reality.  Furthermore, as soon as you request the approach and accept a clearance you have established an intent to fly it. If you did so knowing that the weather station was inoperative I believe you could be held liable for any resulting unpleasantness. But that's a legal question.

@Carlo Felicione, BTW I agree with your answer and upvoted it.  My "semantics game" comment was actually directed at Robert for his comment on your answer.

@Michael Hall, you're quite right, I am asking a legal question, and it's all about semantics, ie: the meaning or intent of the phrase "begin an instrument approach," and "cleared approach." As you mentioned, it's not unusual to be cleared on to a segment of an approach without being cleared for the approach. That's a good example, since 135.225(a)(2) also says that no pilot may "begin" an approach when weather is reporting below mins. But it's a common practice to do just that, and receive clearance if vis improves prior to the FAF. In that case, the approach "began" at "cleared approach."

It's a distinction that could be quite helpful in EMS ops that I'm involved with. Recently the ASOS has been OTS at an airport that is frequently requested. We can legally file if the forecast is VFR, but MEA/MVA are quite high. Being able to descend on a segment of the IAP would be helpful. But I agree that it may very well not meet the intent of the reg. It's interesting to note that when 135.225 was revised in 2001 to allow approaches without weather reporting, a major reason cited was the needs of 135 EMS operators serving smaller communities.


text:
What are the effects of increasing Wing Aspect Ratio?
I can think of only 2 effects that is

The Induced Dag would Reduce with increasing AR of wing.
This goes by the simple formula:

$$ C_{d(i)} = \frac{K \times C_l^2}{\pi \times \text{AR}} $$

As drag decreases as AR increases & lift also decreases as AR increases, so the Aerodynamic Performance (L/D) decreases. To check this we can use another simple formula:
  L= Cl*1/2*ρ*V^(2)*S

&,Cl α 1/(AR)

But what are other effects of increasing AR (including Structural & Aeroelastic Effects)?
Also One can correct me if they feel I am wrong.

Are you looking for aerodynamics only or also for structural and aeroelastic effects?

I would like to know about Structural and Aeroelastic Effects also

Other aerodynamic effect: higher AR -> lower wing chord -> lower Re-Number -> (depending on the speed) higher viscous drag

As drag decreases as AR increases & lift also decreases as AR increases, so the Aerodynamic Performance (L/D) decreases.

Why should lift also decrease? You should keep wing area constant, so lift should also stay constant. This translates into increased L/D.
Generally, an aspect ratio increase has these consequences:

More wingspan, so induced drag will become lower.
Higher wing root bending moment, so structural weight goes up. This has two contributing factors: A higher lever arm of the lift forces and a reduced root thickness because of reduced wing chord. For airliners, this puts the current optimum aspect ratio somewhere around ten, for gliders the optimum is somewhere around 30.
Reduced wing chord which reduces the wing's Reynolds number, increasing viscous drag slightly.
Reduced wing volume, so less fuel can be carried. The fuel requirement drove early jets to lower aspect ratios, which also had the benefit of less complex flaps since wing area was rather large.
The lower chord also reduces the wing's pitching moment, so a smaller tailplane can be employed, or the tail lever arm of the horizontal tail can be shortened.
Higher adverse yaw, which requires a larger vertical tail surface for coordinated flight or the tail's lever arm needs to be increased.

The larger aspect ratio means the span of the wing increases and hence so does the weight of the structure. So, to carry the same load you need a heavier wing, which will decrease the benefit of the lower drag, or you need to choose different materials (metal -> composite) or construction technique (lightening holes in ribs, for example) for the wing to decrease the total weight. It's a tradeoff that you have to balance.

Frontal drag increase, which is not very important on low speeds, but increases dramatically on higher speeds.
Construction weight and resillience - the longer wing, the lower strenghts it can carry. So the long wing is good choice for relatively slow glider (low induced drag - low strenghts and little resillience required), but for higher speeds the cost overcome the benefits.


text:
Do private jets ever fly at FL510? If so, at what occasion?
Most private jets have a service ceiling of 51,000 ft. A service ceiling is the altitude at which a plane can climb no more than 500 ft/min, right? However, on flightradar24 I don't remember having ever seen a jet flying that high, nor at 49,000 ft. The highest-flying jets on flightradar24 are at 47,000 ft but that's very rare, usually 45,000 ft are the highest cruise of jets; usually these are Gulfstreams 650/700, Cessna Citation jets and Global 6000s.
If these private jets have service ceilings of 51,000 ft, why don't they fly that high on flightradar24 and at what occasion would a jet do so (very little remaining fuel e.g.?)?

@Bianfable Thanks for the edit but I think the flightradar24 tag should be added, for the answer may be something like flightradar is less able to show planes that high above the planet, or that pilots at these altitudes make themselves invisible on flightradar for some reason. So perhaps planes do fly there but are invisible on fr24. I don't know.

I don't think it has anything to do with FR24, but if you feel strongly about it, you can edit it back in.

@Bianfable I don't think so either, but it's mentioned in my question often so I'll reedit it in.

Generally when you run right at service ceiling you are near the edge of the operating envelope with little energy margin if things go off the rails, so you are in a relatively dangerous place (also, aerodynamic forces, and damping, are drastically reduced in the thin air, but the inertial mass is the same, which tends to exaggerate overcontrolling/overshoot tendencies).
Less so if you are very light, but business aircraft are usually operating "heavy", in the first part of the trip especially, because the interior weighs so much (empty weight is permanently high) and passenger load is relatively small.
So if you are going to cruise at ceiling you have to be extra careful monitoring speed/mach to make sure you don't run out of energy margin.  If you run through some smooth mountain down wave air, that starts you gently descending, to which the autopilot reacts by pitching up to keep the flight level, and you aren't paying careful attention, you might find yourself with speed decaying toward shaker in very thin air.
You have little thrust margin, and at this point, if you are sliding onto the back side of the power curve, you are in big trouble and need to descend immediately to regain energy.
So if 45000 ft is high enough to clear 99% of the weather, there is little need to go all the way to FL510 when you are much safer, from a general operational perspective, at FL450.  The only motivation to go that high would to be to get clearance over the top of a cell in the tropics (mostly to avoid the hail that can come shooting out the top) without having to divert upwind of it (once you're down near the equator, even 51k may not be high enough and you still have to go around them).

Concorde flew higher because if was a way to limit drag (fuel consumption) at M2, drag being proportional to the square of the speed. Bizjets don't have this problem.

And the bizjet operator may see some theoretical cost saving going there, but it's not worth it, when the boss is sitting in a $100,000 dollar chair in the back and isn't going to care anyway.

As you climb to the tropopause it stops getting colder, so thermodynamic efficiency of the engine does not improve any more, and since you are Mach-limited, you are not flying faster, so propulsive efficiency does not improve either (jet engines get more efficient with TAS). Tropopause is on average at FL360, and rarely above maybe 450, so you shouldn't see much cost saving above that.

@JanHudec At the equator, the tropopause can be as high as FL600 but that's unachievable by current bizjets anyway. The following link may be interesting: https://www.airspacemag.com/daily-planet/after-walking-on-the-moon-150436188/ The link put one record wrong however: the Proteus reached a 19,015 m (62,385 ft) altitude record for horizontal flight while the value in the link, 19,277 m (63,245 ft), is in fact Proteus' absolute altitude record (for a parabola, obviously).

At the other extreme, in the high the arctic in January, it  can come down into the high teens. The U2 cruises at 70000.  But its allowable speed range is about 10kt.


text:
Why airflow travels faster on upper wings?
I have been looking for solutions and come up with two popular explanations:

Conservation of mass / Continuity

For a positive-cambered wing, the streamlines would be compressed/converged at the leading edge's upper part, forcing the airflow to accelerate in order to maintain the same mass flow rate at the entry and exit point.

Air circulation

Air circulates around an airfoil in clockwise direction and hence accelerating the flow on the upper part.
Would like to ask:

Is it appropriate to use conservation of mass / continuity to explain.
What exactly causes circulation on airfoil?

Follow up on question 2, some explanations saying because of the sharp trailing edge, starting vortex is formed which forces the formation of bound vortex. If it is the case, how sharp an edge would lead to a starting vortex, and why it flows from the lower wing to upper wing?
Thanks for helping!

related, if not duplicate https://aviation.stackexchange.com/q/21664/1467 https://aviation.stackexchange.com/q/16193/1467 https://aviation.stackexchange.com/q/8877/1467 https://aviation.stackexchange.com/q/21069/1467

The idea of "circulation" is a confusing one.  It is an abstract concept useful only for some engineering models, but doesn't help with understanding the basics of flight.  I encourage you to read the answer to the linked question and let us know if that clears things up.

If the air is circulating along the surface of your wing, you're not flying, you're stalling.

Static pressure differential is the root cause. Conservation of mass and circulation are mathematical models or descriptions of the end effect rather than physical causes.(Bernouelli's effect is similarly a description of behavior rather than a cause.)
Lower static pressure near the rear of the airfoil and higher pressure at the front is a force and that force accelerates the fluid's mass.
More crudely put there is a modest hole in the air above the rear of an airfoil with some positive angle of attack, created by the airfoil deflecting flow down and so other air rushes in to fill the hole.

It may be easier conceptually to consider the movement of the wing as the
driver of circulation through static air.
If you ever observed the transom of a moving boat, at slow speeds there is a boiling, turbulent flow with water still attached to the boat.  This is fluid viscosity at work.
Now, speed up enough where the boat is going faster than the water can "fill in the hole" and ... a significant reduction in drag.
Wings work in a similar manner, they are literally moving faster than the air can equalize the lower pressure created by its "wake".  The air returns to where the wing was resulting in downwash.
Also downwash is created by the bottom of the wing deflecting air downwards.  The combination of top lift and bottom lift of the wing (and fuselage) create the necessary opposing force against gravity, and it is the top lift that is lost in the stall.
Conservation of mass is obvious, perhaps better explained as conservation of momentum mv, or by action - reaction.  This is suited for air deflected from the bottom of the wing.  Circulation, and creation of lower pressure above the wing, is more about what goes on in the wake of the airflow created by the Angle of Attack.


text:
Is there a physical argument for the Kutta-Joukowski theorem?
I am doing the following exercise:

Consider inviscid, incompressible, steady flow. The Kutta-Joukowski theorem $$L' = \rho_\infty V_\infty\Gamma$$ where

$L'$ is the lift per unit span
$\rho_\infty$ is the freestream pressure
$V_\infty$ is the velocity
$\Gamma$ is the circulation taken around the body

was derived exactly for the case of the lifting cylinder. Equation $L' = \rho_\infty V_\infty\Gamma$ also applies in general to a $2$-dimensional body of arbitrary shape. Although this general result can be proven mathematically, it also can be accepted by making a physical argument as well. Make this physical argument by drawing a closed curve around the body where the closed curve is very far away from the body, so far away that in perspective the body becomes a very small speck in the middle of the domain enclosed by the closed curve.

And here is the solution from the author:

The flow over the airfoil can be syntheized by a proper distribution of singularities, i.e, point sources or vortices. The strength of vortices, added together, give the total circulation, $\Gamma$, around the airfoil. This value of $\Gamma$ is the same along all the closed curves around the airfoil. In this case, the airfoil become a speck on the page, and the distributed point vortices appears as one stronger point vortex with strength $\Gamma$. This is exactly equivalent to the single point vortex for the circular-cylinder case, and the lift on the airfoil where the circulation is taken as the total $\Gamma$ is the same as for a circular cylinder, namely, equation $L' = \rho_\infty V_\infty\Gamma$.

Author's figure:

I understand the author's solution until the part that said:

the lift on the airfoil where the circulation is taken as the total $\Gamma$ is the same as for a circular cylinder.

The equation $L' = \rho_\infty V_\infty\Gamma$ is derived from the circular-cylinder case which we have to do the integral of the pressure distribution over the cylinder surface. But I have not understood why the lift on the airfoil is the same for a circular cylinder like author said.

Can you tell me the book which this is taken from?

@ÉlioPereira Funda "Fundamental of Aerodynamics " by J.D.Anderson Fifth edition. This is one of the problems by the end of chapter 3.

The question as asked in the title is one of the great debates of the discipline of aerodynamics (and you can see by the number of times I've edited this answer that it's still bouncing around in my own head). If you've gotten this far in Anderson and are making this kind of inquiry, you must read McClean. The first 300-or-so pages are devoted basically to answering this very question. Basically: Yes, there is a physical argument (see Peter Kämpf's answer and Anderson 4.5), but saying that lift is caused by circulation is too much of a stretch (which Anderson notes in section 3.16).
Let me turn to your specific example.
Unfortunately, I think Anderson is trying to force the mathematics into the physics rather than start with the physics and describe why the mathematics is a good representation (as McClean sets out to do from the beginning). "Make this physical argument by drawing a closed curve around the body..." basically tells us to make a physical argument by first using a mathematical construction, which to me doesn't actually shed any light on the physics. Further, then we're supposed to be thinking about infinite distances (not physically useful), a "speck on a page" (which has nothing to do with airflow), and really just make use of $\Gamma$ (which is defined purely mathematically) rather than describe a physical flowfield. In short, the closest thing to a "physical" argument here is saying more or less that any two things look the same when viewed from far enough away. This explanation does not seem convincing to me. If you put lots of little vortices around the profile of a cow then reduce it to a "speck on the page," will the cow generate the same lift as an airfoil? To get at the physical generation of lift, we need to remain close to the lifting bodies and determine why their specific geometries create flowfields that produce lift. (Consider, for example, why we need a Kutta condition on an airfoil but not on a cylinder.)
Actually, the mathematical proof (based on conformal mapping) uses the same basic "physical" argument that Anderson makes. Conformal mapping tends to be more of a graduate-level topic, though, so beware of feeling like you need to understand all the underlying math at this point.
In summary, Anderson does not do a good job here of explaining physically why the airfoil has the same lift as the cylinder. If you truly want to understand the physics, read McClean. Anderson's "physical" argument is really just a boiled-down mathematical one.

In a talk I attended the author made the convincing argument that only when the Kutta theorem is fulfilled will flow leave the airfoil parallel to the direction of the trailing edge. Every other value of $\Gamma$ would result in flows that try to flow around the trailing edge, and doing so would be physically impossible without separation. If one tried to calculate flow which follows the contour of a regular trailing edge, suction of a magnitude would be required which could not be created even by a vacuum. Only by fulfilling the Kutta theorem could physically observable flow be mathematically described.
If you replace the airfoil with a cylinder and assume the same inflow and outflow angles as on the airfoil, it will have the same lift. In reality, only a rotating cylinder will show something resembling this kind of flow and now the speeds induced by the central vortex are the same that the surroundings of the rotating cylinder show in a crossflow.

I do not see how it answers the question. What you are describing is the Kutta condition. It selects the correct (for potential flow) value of circulation. However, the Kutta-Joukowski theorem should be valid no matter if the Kutta condition is valid or not. It does not say why circulation is connected with lift.

@VladimirF: You are right. I should had read the question more carefully. I added a new paragraph.

authors: Vladimir F Героям слава


text:
What are these wavy lines on fighters' canopy?
As shown in the images below, what are these wavy lines on the canopy of the aircraft? Do these lines have any utility?
These are mostly captured in videos of cockpit views of carrier-based aircraft.

That top picture is the view from a T45.  The DET cord is designed to rain molten shards of glass down on the pilot, preferably into the neck and eyes (always use your visor!).  I suppose it also has the secondary benefit of weakening the canopy for escape during the ejection sequence, but we were always under the impression that if you pulled the handle you were gonna get burned.

What type of jets are those in the bottom picture?

@dalearn Looks like a BAE Hawk.

The one on the right is a BAe Hawk but the one on the left is the Dassault Alpha Jet operated by QinetiQ.

It is an explosive cord which helps to crack the canopy (pdf!) before the ejection seat has to do this all by itself. When ejection is commanded, the harness is tightened and the canopy is shattered, and only then the real ejection starts. Earlier designs would blow the full canopy off, but this takes longer than shattering it, especially when the canopy is large.
Some seats have their own canopy breakers, but thick, tough canopies which are needed for flying at high dynamic pressure and for surviving bird strikes would slow the seat down too much. The F-16 canopy is 12mm thick in the area above the pilot.
They are used widely for both land- and carrier-based aircraft, and modern designs try to make them less obvious. The F-35 canopy uses just one line down the middle.

Are these chords specific to carrier-based a/cs or I just happened to view the few right ones? i.e. does this class of a/cs need this more than others?

Is that a Chord, Chord, Cord or Cord?  :)

@FreeMan: Thanks, fixed it. Or is it a rope?

@Raj: It could be that the birdstrike requirements for carrier-based aircraft are more demanding, but I don't know anything about this. They are used widely, and the BAe Hawk in your lower picture is mostly used from land.

FYI, it's called MDC, Miniature Detonating Cord.

@Simon … or FLSC - Flexible Linear Shaped Charge.

@Raj - generally, most 0/0 capable ejection-seat capable aircraft have them. Many use these explosives to fracture the canopy prior to seat rocket booster ignition

If it's strong enough to shatter the canopy, how is it safe for the pilot?

@Vader: The explosive sits on the outside, in grooves cut into the outer acrylic layer of the canopy. The linked PDF has a drawing explaining the details.

Its so you don't get dead Geese.

@Vader Its a shape charge that explodes away from the cockpit. The actual MDC sits on the inside of the canopy (you can touch it) in the T45 you have pictured above. Generally what we were told is that the sequence is so rapid that you will invariably travel through shards of molten glass as the canopy fractures above you.

@Vader it's a balance of risk, it's safer than breaking your neck on a solid canopy or remaining in an aircraft that's about to crash.

For completeness, there are also several other mechanisms to make sure the pilot does not "punch through" the canopy, which would likely be fatal.
As well as the detonating cord, many canopies unlock at the front when the ejection handle is pulled.  The airflow gets under the lip of the canopy and tears it off the aircraft.  The hinges at the back are designed to break in this scenario.  Other canopies have explosive charges which detach the canopy.  The Harrier, amongst other examples, might require the pilot to eject at zero or low forward airspeed so utilises detonating cords to make sure that the canopy is shattered.
Some seats also have a rail which extends upwards at the start of the sequence, the top of the rail being designed to shatter the canopy.  The seat then travels up the rail.  The seat itself might also have "canopy buster" spikes fitted at the top such that if the canopy does not detach or shatter, the seat will break through the canopy and the pilot will not be too shredded by the broken plexi-glass.
The ejection sequence is carefully controlled with mechanical timers which, although everything happens fast, make sure that the pilot does not hit the canopy and is ejected clear of the aircraft.  It's very complex, with a sequence of pins being withdrawn, timers started, rockets fired and so on but at least one manufacturer, Martin Baker, has a proud record of never having had a seat failure other than for user or maintenance error.

In some egress systems the timers are pyro delays rather than mechanical -- less to go wrong.


text:
Clarifications to VNAV PTH and SPD INTV during descent and approach
Concerning FMC Update 14.0+, also called “common VNAV”
Scenario: During descent through 8,000 ft, while being radar vectored in the approach, the typical autopilot Flight Mode Annunciators (FMAs) would display: FMC SPD | HDG SEL | VNAV PTH. The next waypoint in the LEGS page has a speed set at 235B and the following waypoint after that is at 10nm with 3000A/180B - though of course you are actually on radar vectoring, the lateral path is just for guidance.
Suppose an unexpected tailwind occurs, and you decide to start the deceleration early by pressing the SPD INTV button and setting a new speed of 210 kts (approximately up speed plus 5-10 kts).

Clarification 1: When SPD INTV is used to adjust the speed, will the autothrottle (A/T) mode remain in FMC SPD?
The FCOM defines MCP SPD as “the autothrottle maintains the speed set in the MCP IAS/MACH display.” However, MCP SPD does not appear as an option within VNAV, hence why FMC SPD is still the correct mode.
Clarification 2: Will the aircraft begin to decelerate towards the new target speed set by SPD INTV, as displayed in the MCP speed window?
The FCOM states that FMC SPD mode means “the autothrottle maintains the speed commanded by the FMC. The autothrottle is limited to the N1 value shown on the thrust mode display.” If SPD INTV is used, does it set a new target speed in the FMC, effectively “overwriting” the target speed in the CDU? Or does the FMC ignore the MCP IAS/MACH value and continue to use the former CDU target speed speed for its calculations?
Clarification 3: As an alternative to using SPD INTV, could selecting and executing the next waypoint in the LEGS page provide the FMC with updated wind information and assist in recalculating the descent point?
Or could a combination if selecting and executing as well as opening the SPD INTV window be an effective solution?
Clarification 4: When refreshing the LEGS page near a deceleration point, is it possible for the deceleration point to completely disappear? (I believe I have observed this on numerous occasions)


text:
What did the controller say during my approach to land (audio clip)?
I recorded this ATC audio clip during a recent flight.
I'm not sure but he may be saying,

6CP, cancel lining clearance......(missing)....traffic will be in position.

Can you complete this transcript? And I don't understand what "cancel lining clearance" is...

Nice question! I think this type of questions are really good to help people understand ATC instructions better in non-standard situations. Can you please keep the recording accessible for the public f̶o̶r̶ ̶a̶ ̶l̶o̶n̶g̶ ̶t̶i̶m̶e̶ / forever?

obligatory "Please hurry with an answer, I am running out of fuel" joke... ;-)

A copy of the audio hosted on SE: https://meta.stackexchange.com/a/333511/375146 (You just need to click "Run code snippet")

Wow that's horrible. I couldn't make any of it out and I've been listening to ATC for some time.

You need to ask ATC to repeat. Otherwise you might not have the opportunity to find out later. Especially if you are approach.

@copper.hat I assume the OP was not piloting the aircraft at the time

@OrangeDog: One hopes...

@copper.hat actually, apparently not

@OrangeDog: Disturbing.

@Joshua same. I'm just a casual visitor but I couldn't understand a single word (native English speaker).

I think this is what the controller is saying:

November Six Charlie Papa, cancel landing clearance. I'll have it back for you in a mile or so, traffic will be in position.

We can infer from this that the aircraft he's talking to (shortened callsign N6CP) has previously been cleared to land. But sometime since the controller decided to have another aircraft to line up and wait "in position" on the runway. The controller is now canceling that landing clearance, because he can't clear an aircraft to land while that aircraft is waiting for departure. And he's letting N6CP know that by the time they get 1 mile closer to the runway, he's anticipating that the waiting aircraft will get cleared for takeoff and N6CP will be cleared to land (again).

Thanks a lot! As you mentioned, I had landing clearance and I've got the instruction from the ATC while I approaching to land. I can understand now!

@gusdyd88 -- if those instructions were addressed to you, you should have asked ATC to repeat them.

The “in a mile or so” surprises me, not being a pilot myself. Is the plane moving slow, about the speed of my car down the highway? Or did he take a little under 5s to cancel a landing clearance only to plan on reinstating it in <30s? It seems like after gusdyd88 asked for clarification ATC would have just responded “I canceled your landing clearance but by now you are re-cleared to land.” Is ATC “just following the rules?” Or maybe “just in case that plane doesn’t move and has a sudden power loss where it stands I don’t want to have to more hastily cancel a landing later” or so?

@CRDrost Yes, both, it's rules and practicality. It's probably a pretty severe violation to be "cleared" to land when there is a known obstruction (like another plane stopped) on the runway. Controller in BIG trouble if something happens. The subtext is also "a command to abort could happen, be ready" but also "continue your approach at your discretion".

Ah, that makes a lot of sense. Thanks!

@CRDrost Two sources of confusion here:  First, this is obviously a small plane--they fly a lot slower than the big boys.  Also, this is near the airport.  Civilian aviation has speed limits when below 18,000'--even when a plane could go a lot faster it's not allowed to.  (Military planes in their training ranges are allowed to go faster--but if they lose the avian lottery they can punch out.)

Anytime communication is addressed to you, you need to understand all of it.
Correct is to either

ask him to repeat, or
read back the part you understood, if you think the rest is unimportant.

Leaving a communications gap creates a hazard.  Nobody minds repeating because they know how scary miscommunication is.
Imagine
"Roger, landing clearence canceled, did not copy the rest, 6CP"
"6 charley papa, turn right heading 220, expedite!"

Very true, but was the communication addressed to the OP or did they just hear it on the frequency?  A canceled clearance requires a read back, and the questioner doesn't seem to have been in a position to do that.

@DaveGremlin From a comment that existed prior to this answer: " I had landing clearance and I've got the instruction from the ATC while I approaching to land."
It seems like that is indeed the case, from the quote at least.

@Baldrickk  - I stand corrected.

How does this answer the question? I am rather new on this site so I was curious if answers like this are permitted; I also didn't want to open a meta post just for it, so I ask here in comments. Because to me the question is "What did the controller say" and not "When I don't understand something in ATC that is addressed to me, what do I do?".

@geisterfurz007 happens all the time where askers open with one thing, then in comments it emerges they are talking about something else entirely.  Often questions are less about "what they literally asked" and more about "what they need to know".  See also XY Problem, Frame Challenge, etc. Often we don't elaborately disclaim those, but some of us bristle at California Warning Label culture.  Speaking of culture, the instant I read your comment I figured your main haunt is StackOverflow.  People are very salty there.  They're much nicer here.


text:
How can I tell from the location of flow separation if an airfoil is stalled?
I am in the process of completing a carbon airfoil for my track car (sedan) and I am looking to determine stall angle visually.  AKA, wool tufts and highway runs with my GoPro pointed upward from the trunklid.
Since I don't really know the angle of incidence at which the air coming off the rear window will hit the wing, visually looking for flow separation is really the only way for me to determine my max angle and work down from there based on the car's balance at the track.
Is there a paper (or common knowledge) regarding over what % of chord length flow separation must occur for the wing to be considered stalled?  Highly cambered airfoil, Selig S1223, 11.5" chord, 66" span.  I have searched for papers on the topic but cannot find anything.
Thanks!

Is there any way for you to measure the amount of downforce you're getting?  When the airfoil stalls that should decrease significantly

Yes, but that would be more involved in terms of adding suspension components than I really want to get.

The boundary layer separates usually at somewhere around 10-15% of chord near stall AOA (which is where you place vortex generators when using them to delay stall AOA).  Separation will start at the trailing edge and move forward.  If it was me I'd tuft the entire lower surface from just behind the LE, maybe 10% so you can really see everything that's going on.  Doesn't hurt to put too many on.

The location of separation is very hard to predict accurately and the range can be quite large when comparing leading-edge and trailing-edge stall. So tuffing the entire lower surface is indeed recommended.
XFoil can get you a good initial guess though.
The stall behavior can differ significantly at different Reynolds numbers (read speed). In order to be able to map you video to GPS data, you might want to include a clock in your image, so you can limit the amount of test runs.
As a sidenote, I assume you're also considering a Gurney Flap?


text:
How much revenue do seat-back displays generate?
Many airlines are toying with the idea of getting rid of seat-back displays, how much revenue did those seat-back displays bring in the form of in-flight advertisement? I would presume for a long haul flight the revenue might be more than what it is for a mid-range flight.

5 hours is short-range?

OK - my bad - short flight < 3 hours.

I removed reference to "short-range" flight

I don't have the precise numbers but those displays take up power (==fuel) and weight and need to be serviced. I'm sure that there is some tipping point between profit from adds and running cost that they are very carefully analyzing.

and airline that bombards me with advertising using their displays will find that I quickly position a towel or something to block it out, and probably won't fly that airline again...

I personally have seen very little advertising on seat-back displays, so I would suspect it's not much.

@cpast After crossing the Pacific a few times, yeah, 5 hour flights start to feel much shorter. - haha - Unless, of course, you're in an uncomfortable seat. Then they can feel longer than the 15 hour flights.

Honestly, I'd be surprised if advertising revenues ever neared the operational cost of the IFE systems, especially since, as GdD mentioned, there's usually not all that much advertising on them. Personally, I've mostly just seen advertising on them during boarding/deplaning when their entertainment functions usually aren't enabled.

OP: can you point to some sources for your assertion that "many airlines are..."

@CGCampbell I'm pretty sure Delta is considering this and I think it has even already done so on some of its fleet, opting to deliver content to passengers on their own devices over wi-fi instead. Southwest follows that model exclusively, but I don't think they ever had IFE systems to start with. There are a couple of articles about this here and here.

According to this article from September 2014:

Thales estimates total revenue from IFE at \$17.6m a year for
  small/mid-sized carriers, up to \$397m a year for large carriers.

But that's for all services, not just advertising. The only specific mention I could find of advertising was this 2012 quote attributed to the CEO of Thompson Aerospace:

Somewhere in the middle of the second question about advertising on
  the system he said: “… and we are generating somewhere between \$20,000
  and \$30,000 per month per plane with the IFE.”

The revenue is apparently generated in a similar way to internet advertising:

[...] every 2 weeks the airline and the advertising service bureau receives
  a summary of the requests/viewings and the airline gets a check. At 1
  to 5 cents per click, the tally adds up as daily flights, sometimes
  short as an hour, record revenue and usage every passenger, every
  flight, every day, and every month

However, there's no information in the article about which airlines actually use that specific service, and I have no idea if those numbers are typical or not, or even exaggerated.
If you need more information than that and you can afford USD 4,650 to find out, this looks like a comprehensive report on the IFE industry.


text:
In the compressor characteristic, what does this value represent?
$$ ? = \frac{m_t* \sqrt{T_3}}{P_3}$$
All I know is that it is a material property of a turbine, it has something to do with the mach number, and is used for calculating $W_T$. It's kinda similar to the corrected mass flow rate but I guess I'm wrong.
I don't have any books on aerodynamics and I feel kinda lost...

In German it is called "Ähnlichkeitsparameter", a comparison parameter for the compressor state. This one in particular is a non-dimensional mass flow comparison parameter. To be more correct your equation is actually missing the gas constant "R" under the square root (together with temperature) to make it non-dimensional. This non-dimensional mass flow factor makes it possible to compare different scenarios like high altitude and high speed at low temperatures as well as low speed, sea level conditions. Just like we do with the lift coefficient on wings.
It's not a material property of any particular engine or compressor, just an operating point with a dimensional mass flow m (kg/s), T3 being the total pressure (K) at station 3, P3 being the total pressure at station 3 (Pa) at that time. Station 3 is entry into combustion chamber.
This factor usually comes up in compressor maps (in German "Verdichterkennfeld"), where you plot the compression ratio (pi) on the y-axis over this mass flow parameter on the x-axis. These maps can tell you how much the compressor can compress the air without stalling.
https://en.wikipedia.org/wiki/Compressor_map

Hi Jan, thank you so much for answering the question! Really helped me out a bunch.